Field of the Invention
The present invention relates to an electric double-layer capacitor.
Description of the Related Art
Electric double-layer capacitors using non-aqueous electrolytic solution are able to store a large amount of energy because the high electrolytic voltage of their solvent allows for high withstand voltage. In recent years, electric double-layer capacitors are seeing their applications expand, while facing strict, demand for devices. To be specific, they are required to ensure a level of reliability backed by operating voltages higher than 2.5 V (such as 2.7 V) and also by small characteristic changes at low temperatures (e.g., −40° C.) and at high temperatures (e.g., 85° C.).
When it comes to improving the low-temperature characteristics (reducing the internal resistance), for example, Patent Literature 1 proposes an electrolytic solution whose solute is spirobipyrrolidinium salt and whose solvent comprises propylene carbonate, ethylene carbonate, and dimethyl carbonate mixed together at certain ratios, for the purpose of reducing the internal resistances of electric double-layer capacitors. Also, Patent Literature 2 proposes an electrolytic solution for electric double-layer capacitors offering excellent low-temperature characteristics, whose solute comprises a mixture of quaternary ammonium tetrafluoroborate and quaternary ammonium hexafluorophosphate, and whose solvent comprises ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate mixed together at certain ratios. Furthermore, Patent Literature 3 proposes an electrolytic solution for electric double-layer capacitors characterized by low viscosity and high electrical conductivity even at low temperatures of −30 to −40° C., whose solute is a pyrrolidinium ionic liquid and whose solvent comprises ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate mixed together at certain ratios. Patent Literature 4 proposes an electrolytic solution for electrochemical elements whose solute is imidazolium salt, wherein the electrolytic solution does not deposit, solidify, etc., even at extremely low temperatures.